(1) Field of the Invention
The present invention relates in general to current source circuits, and particularly to a set of current source circuits exhibiting adapting or trimming capabilities; even more particularly to sets of multiple matched current source circuits used in LED drivers manufactured as semiconductor integrated circuits.
(2) Description of the Prior Art
Recent development trends in electronic devices for modern flexible and versatile telecommunications and data processing equipment combine various all-purpose applications into one single device often additionally featuring image displaying capabilities thus fostering an increased application of high quality displays. These displays are also enhancing usability by offering easy to use man-machine interfaces, thus playing an important role in customers' acceptance of the equipment. Such displays are nowadays mostly from the LCD (Liquid Crystal Display) type fabricated in STN (Standard Twisted Nematic) or TFT (Thin Film Transistor) technology needing additional back-lighting but of late often also made as LED (Light Emitting Diode) displays in form of self-luminescent OLED (Organic LED) and PLED (Polymer LED) devices. Ever more displays, being capable to exhibit their own luminosity without extra light sources are preferred. Worth being mentioned in this context are also so-called Surface conduction Electron Emitter Displays (SEDs), High Dynamic Range (HDR) displays, Field Emission Displays (FED) and most recently presented, QDLED-Displays making use of Quantum Dot crystals. Currently in common use are OLED and PLED displays however, in PMOLED (Passive Matrix Organic) LED and AMOLED Active Matrix Organic) LED structure forms. As is well know for such displays, optimum performance especially with highbrightness LEDs is achieved only when the LEDs are driven by current sources rather than by voltage sources. Most modern integrated LED driver circuits are therefore utilizing multiple sets or arrays of almost identical current sources. Due to manufacturing variations during the semiconductor production process a strict conformity to the design values is never reached and in order to keep the errors small an often practiced method is to make the relevant transistor structure dimensions much larger than technological design rules would require, thus matching the resulting current sources better to each other and/or to other external actualities by minimizing the relative errors, although wasting lots of chip area which is therefore a rather costly method. Another viable way to reduce manufacturing errors is to trim during a subsequently added separate production step crucial transistor dimensions of the current sources e.g. by laser trimming, in order to guarantee a prescribed uniformity within one set and also allowing respectively first making possible the principal tracking of pixel specifications from the adjoint LED display e.g. for color hue, brightness, saturation values and the like. These quasi-static matching of the corrected current sources is then dynamically superimposed by individually controlled appropriate display driver signals enabling the current sources actual main task of driving the LED pixels for showing the real information.
As can already be seen from the above both methods to achieve a better matching of the current sources are really expensive, either because of the surplus chip area consumed and/or because of the excessively time consuming trimming and the costly laser and measurement equipment necessitated by that extra production step.
Circuits for current sources exist as prior art in numerous variants, they are also utilized in form of current mirrors: the most suitable and well known basic forms in the art are designated as Widlar and Wilson current sources, whereby said basic Widlar current source made up of two transistors shall be used in case of our preferred embodiment of the invention taken here as showcase and described later on in greater detail. More advanced circuits are realized as Cascode current sources or Temperature-Stabilized current sources directly derived from these basic forms; as more advanced current mirrors may be mentioned Cascode current mirrors and Buffered current mirrors, also High Swing, Stacked, Beta Helper added and Super Wilson subtypes, all these exhibiting much more complex structures, but also always showing the same underlying circuit basics so that the principles of the invention as explained later on can easily be applied to all these circuits. These same or slightly modified circuits are also used for current Sink purposes or for current source as Load applications, especially in form of Complementary current source Loads; even more advanced as current source or current sink Load Inverters, which are often used in memory driver circuits, especially for modern nonvolatile memory technologies such as Magnetoresitive (MRAM) and Ferroelectric (FRAM) Random Access Memory (RAM) technologies. Important also to bear in mind: all these current source, sink or load circuits mentioned above can be implemented either as Bipolar Junction Transistor (BJT) or as Metal-Oxide Semiconductor Transistor (MOST) devices. In both technologies the discrete parameters of the current sources are defined by the structural dimensions of the transistors involved, mainly the dimensions of the emitter or gate areas, which are thus crucial for the dimensioning of the circuits.
Modern industrial applications making use of such types of Current Source/Mirror/Sink/Load circuits can be found in many fields; one important example is e.g. within an electrical tomography system, whereby accuracy and stability of these circuits as crucial components in these systems are playing an important role because of its direct influence on the exactitude of the results for medical diagnostics.
A variety of solutions is found in the prior art for controlling structural device features in an attempt to simultaneously reach the two competing goals namely manufacturing accuracy for matching multiple devices and cost effectiveness in production. Nevertheless, additional improvements in both fields are desired and continued improvements in these areas are needed. It is therefore a challenge for the designer of such circuits to achieve an even more flexible solution which is also furnishing a higher accuracy. There are various patents referring to such solutions.
U.S. Pat. No. 4,766,366 to Davis presents a trimmable current source for use with low voltage circuitry which includes a plurality of trimming networks. A voltage-divider circuit is connected to the trimming networks. Each of the trimming networks includes a resistor in an isolated epitaxial region series connected to a zener diode. A programming signal, having a voltage level which would normally damage the low voltage circuitry can be applied to the junction of the resistor and zener diode, and to the isolated epitaxial region containing the resistor of the trimming network to be programmed without damage to the low voltage circuitry.
U.S. Pat. No. 4,967,140 to Groeneveld et al. discloses a current source arrangement in which N configurations of N+1 transistor configurations (TC_1 to TC_N+1) comprising control transistors (T_1 to T_N+1) and control inputs (CI_1 to CI_N+1) are connected to N outputs (1, 2, . . . N) by means of a switching network in accordance with a cyclic pattern N. The remaining configuration is connected to a correction circuit which includes a reference-current-source for adjusting the control voltage of the control transistor via the control input of the relevant transistor configuration, in such a way that the output current of the relevant configuration becomes equal to that of the reference-current-source.
U.S. Pat. No. 5,581,209 to McClure teaches an adjustable current source wherein an output driver circuit for an integrated circuit is disclosed, where the output driver drives an output terminal with a high logic level having a voltage limited from the power supply voltage of the integrated circuit. The limited voltage is provided by applying a limited output high voltage to an output buffer, such that the drive signal applied to the gate of the pull-up transistor in the output driver is limited by the limited output high voltage applied to the output buffer. A voltage reference and regulator circuit for generating the limited output high voltage is also disclosed, and is based on a current mirror. The sum of the current in the current mirror is controlled by a bias current source, which may be dynamically controlled within the operating cycle or programmed by way of fuses. An offset compensating current source adds current into the reference leg of the current mirror to eliminate the development of an offset voltage in the current mirror, and the limited output high voltage is shifted by the threshold voltage of the pull-up drive transistor by way of a threshold shift circuit.
U.S. Pat. No. 6,999,048 to Sun et al. describes an integrated data driver used in a current-driving display device which includes a digital-to-analog current converter for transforming a digital signal into an analog current signal, and a plurality of sets of data driving circuits for driving a plurality of corresponding data lines, whereby each set of data driving circuits includes a current-copying/reproducing module and a control circuit. The current-copying/reproducing module is used to store a predetermined voltage for conducting the analog current signal in a transforming/storing status and to conduct a reproducing current signal, which is generated by the predetermined voltage, to the corresponding data line in a reproducing/sustaining status. The control circuit is electrically connected between the digital-to-analog current converter and the current-copying/reproducing module for providing a switch between the transforming/storing status and the reproducing/sustaining status.
In the prior art, there are different technical approaches to achieve the goal of a higher accuracy production and/or for easier trimming methods of the integrated current source circuits. However these approaches use often solutions, which are somewhat technically complex and therefore also expensive in production. It would therefore be advantageous to reduce the expenses in both areas.